Porous monolithic materials are used in a variety of applications such as filtration, adsorption and catalysis, among others. Such materials are often mounted in and contained in intimate contact with a supporting structure such as a containing tube, which both supports and protects the porous medium, and for some applications, confines the flow of a liquid or gas. For proper function of a particular process, it can be important to confine the fluid (or gas) flow through the porous monolith and to minimize flow around the porous monolith, which may occur when the fluid flows in gaps present between the porous monolith and its supporting structure. Leakage through gaps between the porous monolith and its supporting structure can result in insufficient or poorly controlled contact between the porous material and the fluid (or gas) and any components present therein, diffusion broadening of chromatographic peaks, inadequate adsorption or catalysis, etc. In order to avoid these problems, it is necessary to provide a liquid or gas tight contact between the edges of the porous monolith and its supporting structure so that contact between the fluid and its components with the porous monolith can be controlled as desired.
Depending on the application and the choice of specific materials appropriate to that application, a variety of assembly processes may be used to make structures that are suitable for applications which include a porous monolith associated with a support structure. However, many of these structures suffer from excessive leakage around the edges due to shrinkage of the porous monolith away from its associated support. For example, inorganic materials such as glass or ceramic are frequently used for their resistance to solvents and high-temperature. The manufacture of structures using these materials frequently requires the use of high-temperature processing steps. For this and other reasons, there is often significant shrinkage of the porous material during manufacture. The use of a rigid support material may result in the appearance of excessive gaps at the inside wall after the completion of all manufacturing steps, and cause the porous material to be unsecured with respect to its supporting structure.
One solution to these problems is to provide a shrinkable support that can contract to provide close contact with the contained or associated porous monolithic material. Shrinkable polymers are well-known and widely-used. In sheet form, they are used in vacuum forming and in “shrink-wrapping.” In tube form, they are frequently used to form a protective, insulating and/or supportive outer layer of near-cylindrical parts, especially for electrical applications. However, such polymer materials cannot be used in many applications, because the thermal and/or chemical resistance of the material is not compatible with the application.
A common technique in precision metal fabrication is to make an outer part with an inner dimension just slightly smaller than the outer dimension of an inner part. By heating the outer part, its dimensions can be expanded just enough to allow insertion of the inner part. On cooling, the two parts have an “interference” fit which provides intimate contact without leakage and a firm connection without a bonding agent. However, such techniques are only suitable if the dimensions of the two parts to be assembled can be held to extremely tight tolerances, which may not be possible with porous monolithic materials.
Similar techniques are used to create electrical or other metallic feedthroughs in the glass walls of vacuum tubes, light bulbs and the like. Metal rods or wires are inserted through holes or glass tubes. The glass is heated until it softens and the holes or gaps shrink around the metal to form a vacuum-tight seal. Thus, although using glass as a shrinkable medium to provide sealing against metal parts is known in the art, it has not been used with porous monolithic materials to provide sealing between the containing or supporting walls and the porous monolith. In contrast, glass has been used as a container and mold for producing porous monoliths formed by sol gels, but upon drying and calcination, the porous monolith becomes loose in the support. This problem has not been solved.
Sol-gel processing of glasses has been used for the manufacture of optical fibers having an overcladding tube. For example, U.S. Pat. No. 5,922,099 to Yoon, et al. and U.S. Patent Application Publication No. 2003/0148053 to Wang, et al. describe casting sol-gel materials into a tubular mold to form the overcladding tube for an optical fiber and a sol-gel-derived rod, comprising a cylindrical core portion and a tubular cladding portion around and concentric with the core portion. U.S. Pat. No. 6,080,339 to Fleming et al. describes an extrusion process to extrude sol-gel material for the purpose of making overcladding tubes, substrate tubes, and optical fibers themselves. The extruded tubes are then subjected to the usual processing conditions, including heating for preparation of optical fibers. However, these references do not describe methods for providing a porous monolithic core having liquid tight contact between the porous monolith and the containing walls that is suitable for use in analytical or preparative devices, or the like. In fact, the processes utilized in manufacture of optical fibers would be incompatible with retention of porosity in porous monolithic materials, as the presence of pores would reduce the optical clarity and transmittance of the fibers, and therefore, is an undesirable feature in optical fibers.
Sol-gel processing to form a porous monolith inside capillaries is also known. However, the fused silica capillaries have a very high melting temperature, and heating the capillary and porous monolith to a sufficiently high temperature to shrink the capillary down to maintain contact with the porous monolith as it forms and is further processed (e.g., dried and calcined) would destroy the porosity of the monolith, just as in the case of preparation of optical fibers. Therefore, alternative solutions have been sought. For example, U.S. Pat. No. 6,562,744 to Nakanishi et al. describes a process for making capillary chromatographic columns wherein the porous material inside the capillary is allegedly in liquid tight contact with the capillary by virtue of an affinity of the capillary walls for the gelling silicate components in the porous material. This patent also describes using a shrinkable PTFE capillary to form a liquid tight contact between the porous material and the capillary. However, PTFE does not provide sufficient support for fragile porous monoliths formed by sol gel methods, and cannot be heated to a temperature sufficient to calcine the sol gel monolith and remove organic contaminants. Further, the patent teaches that special treatments of the capillary inner surface are required.
U.S. Pat. No. 6,531,060 to Nakanishi et al. describes a similar process for forming a porous monolith inside a fused silica capillary. U.S. Patent Application Publication No. 2003/0213732 to Malik et al. also describes chemical anchorage of the monolith to the capillary walls. In addition, U.S. Pat. No. 6,783,680 to Malik describes the preparation of a sol-gel stationary phase formed inside fused silica capillary tubing and reports that the sol gel is chemically bonded to the capillary walls as a result of condensation reaction with the silanol groups on the capillary inner surface. The capillary can be further treated with heat up to 350° C. However, the above described techniques are only useful when working with small dimensions such as exist in capillary tubing, in which diameters are generally less than 1 mm and typically less than 0.1 mm. For example, the largest diameter fused silica capillary tubing utilized by Malik was only 0.25 mm in diameter, and such small diameters limit the applications and utility of the method.
Another difficulty encountered in minimizing leakage or gaps between a porous monolith and its containing structure, especially when prepared using sol-gel methods, is that processing introduces shrinkage or cracking during the drying step of the fabrication process. Approaches to reduce the cracking have been attempted, focused on increasing the pore sizes of the monolith to reduce the capillary stresses generated during drying. For example, U.S. Pat. No. 5,023,208 to Pope describes subjecting the gel to a hydrothermal aging treatment, which reportedly causes silica particles to migrate and fill small pores in the porous gel matrix, and increase the average pore size. U.S. Pat. No. 6,620,368 to Wang describes that the density of the gel at the end of the first stage of liquid removal process corresponds to a shrinkage in the linear dimension of between about 15% and 35%. However, these additional processing steps are time consuming and the products are expensive to manufacture.
U.S. Pat. No. 6,210,570 to Holloway describes a method for preparing a chromatography column containing a hydrosol with minimal shrinkage, wherein the hydrosol has a first volume, and after being induced to produce a monolith has a second volume that is at least about 95% of the first volume. The hydrosol is described as having a SiO2 concentration of less than about 5 g/ml, or between about 3 g/ml and about 5 g/ml. The patent further states that a balance should be achieved between syneresis prevention and producing a brittle silica product due to too low a SiO2 concentration. This reduction in “syneresis,” or the shrinkage in volume as a hydrosol progresses to a hydrogel, is reported to avoid the resolution problems caused when mobile phase effectively bypasses portions of the stationary phase, resulting in poor separation of components during chromatographic separation. However, these processes for producing a porous monolith with reduced shrinkage sacrifice control of the gel composition, porosity and pore size distribution, and the porous monolithic materials produced lack in mechanical strength.
Accordingly, there remains a need for a shrinkable support material that can be applied to the exterior surface of porous monolithic materials to minimize gaps, provide more intimate contact and reduce leakage.